Supported liquid membranes have been widely studied for the separation and concentration of a variety of compounds and present many potential advantages over other separation techniques. In a supported liquid membrane, the pores of a supporting membrane are impregnated with a particular liquid, and transport of the permeating species occurs by a solution diffusion mass transfer mechanism. The diffusion of species in liquids is generally faster than diffusion in solids, and the permeability across supported liquid membranes is typically greater than that achieved using solid membranes.
Supported liquid membranes are typically fabricated by either direct immersion, or various pressure and vacuum methods. In direct immersion, immobilization of the membrane liquid takes places by contacting or immersing the supporting membrane and allowing the supporting membrane to soak up the membrane liquid. In pressure methods, immobilization is achieved by contacting a porous membrane and a membrane liquid, and applying typically nitrogen pressure to force the membrane liquid to flow into the pores of the membrane. The vacuum method operates by submerging the porous membrane in a volume of membrane liquid and applying a vacuum to the membrane in order to evacuate the membrane pore and draw in the membrane liquid. The pressure and vacuum methods are typically utilized on supporting membranes generally classified as macroporous with a pore sizes of at least greater than 50 nanometers, and typically in the hundreds of nanometers, and success is gauged either by the visual presence of membrane liquid in all pores of the supporting membrane or by the presence of membrane liquid being forced through the lower pressure membrane surface. Pressure and vacuum methods have also been particularly applied to membrane liquids of relatively high viscosity, such as ionic liquids. See e.g., Hernandez-Fernandez et al., “Preparation of supported ionic liquid membranes: Influence of the ionic liquid immobilization method on their operational stability,” Journal of Membrane Science 341 (2009); see also Neves et al., “Gas permeation studies in supported ionic liquid membranes,” Journal of Membrane Science 357 (2010).
The current methods of supported liquid membrane fabrication suffer from a number of shortcomings, such as the unsustainable loss of membrane liquid in macroporous supporting membranes under applicable operating conditions. This membrane liquid loss in-situ is typically attributed to a differential pressure across the macroporous membrane in excess of a capillary force maintaining the membrane liquid immobilized, and places limits on the differential pressure that the supported liquid membrane can sustain. This maximum differential pressure is related to the maximum pore size of the membrane, the pore structure, the interfacial tension of membrane liquids, and the contact angle, among other factors. See e.g., Zhao et al., “Membrane liquid loss mechanism of supported ionic liquid membrane for gas separation,” Journal of Membrane Science 411-412 (2012).
On the other hand, gas separation membranes which operate in the absence of a supported liquid typically consist of a dense film, through which permeates are transported by diffusion under the driving force of a pressure or concentration. These dense film membranes may be significantly more tolerant of differential pressures, however they suffer from a permeability-selectivity tradeoff, where generally as permeability improves the selectivity declines, and vice versa. It would be advantageous to provide a method whereby a membrane liquid could be placed within the pores of a supporting membrane having a dense, skin surface, so that a highly permeable dense layer could be coupled with a highly selective membrane liquid, the permeability-selectivity tradeoff could be greatly reduced, and a differential pressure across the supported liquid membrane could be applied with mitigated concern toward its relation to a capillary force. A limitation of polymeric membranes for gas and vapour transport is the so-called permeability-selectivity trade-off, first reported by Robeson et al. in 1991 and then further updated. This trade-off basically allows that if one is looking for a new material with a higher permeability, the price to pay is a lower selectivity, and vice versa.
Additionally, the membrane liquid layer thickness is of significant concern in supported liquid membranes. Typically, the immersion, pressure and vacuum methods applied to porous supporting membranes offers little control over the resulting liquid layer thickness, and during fabrication the liquid typically extends throughout all pores of the supporting membrane. As a second step, the supported liquid is usually allowed to drain under the influence of gravity or some other force from those pores exerting an insufficient capillary force, so that the final layer thickness and location becomes largely a function of local pore size, pore configuration, the viscosity of the membrane liquid, and the surface tension of the liquid. This liquid layer thickness generally has an inversely proportional influence on permeability, and correspondingly the permeance of the supported liquid membrane suffers, or at a minimum is poorly controlled during fabrication. It would be advantageous to provide a method whereby the membrane liquid could be distributed within the pores of a supporting membrane in a relatively thin, continuous fluid layer, as opposed to a layer which generally extends throughout the interconnected pores of the supporting membrane.
Additionally, significant work is underway to mitigate the relative instability and short lifetimes of supported liquid membranes by utilizing ionic liquids as the membrane liquid. Careful selection of the cation and anion makes possible minimal vaporization losses, and their generally high chemical and thermal stability, high ion conductivity, and high solvent power provide additional advantages. The large diversity of ionic liquids is a result of the nearly unlimited combination of cations and anions which can permit very precise tuning of the ionic liquid solvation properties, density, viscosity, melting point, and even conductivity, enabling the rational design for specific applications. See e.g., Ilconich, et al., “Experimental investigation of the permeance and selectivity of supported ionic liquid membranes for CO2/He separation at temperatures up to 125° C.,” Journal of Membrane Science, 41-47, (2007); see also Pennline, et al., Progress in carbon dioxide capture and separation research for gasification-based power generation point sources, Fuel Process. Technol. 897-907, (2012). However, ionic liquids possess relatively high viscosities, and the preparation of supported ionic liquid membranes using either immersion, pressure, or vacuum applied to a supporting membrane having a dense layer is difficult. It would be particularly advantageous to provide a method where a relatively high viscosity ionic liquid could be placed within the pores of a supporting membrane having a dense, skin surface, so that the ionic liquid could be distributed within the pores of the supporting membrane in a relatively thin, continuous fluid layer, and so that a highly selective ionic liquid could be coupled with a permeable dense layer, greatly mitigating the permeability-selectivity tradeoff of gas separation membranes, and greatly reducing concerns stemming from the relationship between a differential pressure and the capillary forces within the pores in an operating environment.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.